CHEMICAL SPECIATION OF ENVIRONMENTALLY SIGNIFICANT HEAVY METALS WITH INORGANIC LIGANDS

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1 Pure Appl. Chem., Vol. 77, No. 4, pp , DOI: /pac IUPAC INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY ANALYTICAL CHEMISTRY DIVISION* CHEMICAL SPECIATION OF ENVIRONMENTALLY SIGNIFICANT HEAVY METALS WITH INORGANIC LIGANDS PART 1: THE Hg 2+ Cl,OH,CO 3 2,SO 4 2, AND PO 4 3 AQUEOUS SYSTEMS (IUPAC Technical Report) Prepared for publication by KIPTON J. POWELL 1,, PAUL L. BROWN 2, ROBERT H. BYRNE 3, TAMÁS GAJDA 4, GLENN HEFTER 5, STAFFAN SJÖBERG 6, AND HANS WANNER 7 1 Department of Chemistry, University of Canterbury, Christchurch, New Zealand; 2 Australian Sustainable Industry Research Centre, Building 4W, Monash University, Gippsland Campus, Churchill VIC 3842, Australia; 3 College of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, FL , USA; 4 Department of Inorganic and Analytical Chemistry, University of Szeged, P.O. Box 440, Szeged 6701, Hungary; 5 School of Mathematical and Physical Sciences, Murdoch University, Murdoch, WA 6150, Australia; 6 Department of Inorganic Chemistry, Umeå University, S Umeå, Sweden; 7 Swiss Federal Nuclear Safety Inspectorate, CH-5232 Villigen, Switzerland *Membership of the Analytical Chemistry Division during the final preparation of this report: President: K. J. Powell (New Zealand); Titular Members: D. Moore (USA); R. Lobinski (France); R. M. Smith (UK); M. Bonardi (Italy); A. Fajgelj (Slovenia); B. Hibbert (Australia); J.-Å. Jönsson (Sweden); K. Matsumoto (Japan); E. A. G. Zagatto (Brazil); Associate Members: Z. Chai (China); H. Gamsjäger (Austria); D. W. Kutner (Poland); K. Murray (USA); Y. Umezawa (Japan); Y. Vlasov (Russia); National Representatives: J. Arunachalam (India); C. Balarew (Bulgaria); D. A. Batistoni (Argentina); K. Danzer (Germany); W. Lund (Norway); Z. Mester (Canada); Provisional Member: N. Torto (Botswana). Corresponding author Republication or reproduction of this report or its storage and/or dissemination by electronic means is permitted without the need for formal IUPAC permission on condition that an acknowledgment, with full reference to the source, along with use of the copyright symbol, the name IUPAC, and the year of publication, are prominently visible. Publication of a translation into another language is subject to the additional condition of prior approval from the relevant IUPAC National Adhering Organization. 739

2 740 K. J. POWELL et al. Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg 2+ Cl,OH,CO 3 2, SO 4 2, and PO 4 3 aqueous systems (IUPAC Technical Report) Abstract: This document presents a critical evaluation of the equilibrium constants and reaction enthalpies for the complex formation reactions between aqueous Hg(II) and the common environmental inorganic ligands Cl, OH, CO 3 2, SO 4 2, and PO 4 3. The analysis used data from the IUPAC Stability Constants database, SC-Database, focusing particularly on values for 25 C and perchlorate media. Specific ion interaction theory (SIT) was applied to reliable data available for the ionic strength range I c 3.0 mol dm 3. Recommended values of log 10 β p,q,r and the associated reaction enthalpies, r H m, valid at I m = 0 mol kg 1 and 25 C, were obtained by weighted linear regression using the SIT equations. Also reported are the equations and specific ion interaction coefficients required to calculate log 10 β p,q,r values at higher ionic strengths and other temperatures. A similar analysis is reported for the reactions of H + with CO 3 2 and PO 4 3. Diagrams are presented to show the calculated distribution of Hg(II) amongst these inorganic ligands in model natural waters. Under typical environmental conditions, Hg(II) speciation is dominated by the formation of HgCl 2 (aq), Hg(OH)Cl(aq), and Hg(OH) 2 (aq). Keywords: chemical speciation; heavy metals; environmental; ligands; stability constants; Division V. CONTENTS 1. INTRODUCTION 2. OBJECTIVES 3. SUMMARY OF RECOMMENDED VALUES 4. Hg(II) SOLUTION CHEMISTRY 5. DATA EVALUATION METHODS 5.1 Data evaluation criteria 5.2 Methods for numerical extrapolation of data to I m = 0 mol kg Criteria for the assignments: Recommended and Provisional 6. EVALUATION OF EQUILIBRIUM CONSTANTS (HOMOGENEOUS REACTIONS) 6.1 The Hg 2+ OH system Formation of HgOH Formation of Hg(OH) 2 (aq) Formation of Hg(OH) 3, Hg 2 OH 3+, and Hg 2 (OH) The Hg 2+ Cl system Formation of HgCl Formation of HgCl 2 (aq) Formation of HgCl 3 and HgCl 2 4

3 Chemical speciation of environmentally significant heavy metals Other binary Hg(II) chloro complexes 6.3 The Hg 2+ OH Cl system: Formation of HgOHCl(aq) 6.4 The Hg 2+ CO 2 3 system 6.5 The Hg 2+ PO 3 4 system 6.6 The Hg 2+ SO 2 4 system 7. SPECIATION IN 2-COMPONENT SYSTEM: H + L 7.1 The H + CO 2 3 system 7.2 The H + PO 3 4 system 8. EVALUATION OF EQUILIBRIUM CONSTANTS FOR HETEROGENEOUS REACTIONS 8.1 The Hg 2+ OH system: Solubility of HgO(s) 8.2 The Hg 2+ Cl system: Solubility of HgCl 2 (s) 8.3 The Hg 2+ CO 2 3 system: Solubility of HgCO 3 2HgO(s) 8.4 The Hg 2+ PO 3 4 system 9. EVALUATION OF ENTHALPY DATA (HOMOGENEOUS AND HETEROGENEOUS RE- ACTIONS) 9.1 The Hg 2+ OH system 9.2 The Hg 2+ Cl system 10. SPECIATION IN MULTICOMPONENT SYSTEMS: Hg 2+ H + Cl CO 2 3 PO 3 4 SO Freshwater in equilibrium with CO 2 (g) 10.2 Freshwater with varying chloride concentrations 10.3 Summary REFERENCES APPENDIX 1A Stability constants and equilibrium constants Nomenclature for stability constants APPENDIX 1B Complex formation by polyvalent anions (SO 2 4, PO 3 4, CO 2 3 ) APPENDIX 2 Selected equilibrium constants APPENDIX 3 SIT plots for Hg 2+ L systems APPENDIX 4 Equilibrium data for the systems H + CO 2 3 and H + PO INTRODUCTION This review is the first in a series relevant to speciation of heavy metal ions in environmental systems of low ionic strength. The series will provide access to the best possible equilibrium data for chemical speciation modeling of reactions of heavy metal ions with the major inorganic ligands. The metal ions and ligands selected for review are: Hg 2+, Cd 2+, Cu 2+, Pb 2+, Zn 2+ and Cl, OH, CO 3 2, SO 4 2 and PO 4 3, respectively. To enable speciation calculations on these systems, recommended values for the H + CO 3 2 and PO 4 3 systems are also reported. Chemical speciation modeling for labile systems is based on the assumption that all component and derived species are in equilibrium and that reliable stability constants are available at the applicable ionic strength and temperature. The validity of these assumptions is often uncertain. Further, full details of component (stoichiometric) concentrations are required. Despite these factors, modeling has definite value in interpretation or simulation of environmental processes. It is often the only option as the necessary sensitive, selective, and noninvasive analytical techniques for measuring metal ion and metal complex concentrations are still, to a great extent, unavailable.

4 742 K. J. POWELL et al. Detailed knowledge of chemical speciation is essential to a full understanding of bioavailability and toxicity of heavy metal ions, and to their adsorption, sedimentation, and transport phenomena in soils, rivers, and aquifers. The optimization of many industrial chemical processes, as in hydrometallurgy and pulp and paper processing, relies on a detailed understanding of speciation in often-complicated multicomponent, multiphase systems. 2. OBJECTIVES This review is concerned with the Hg 2+ Cl, OH, CO 2 3, SO 2 4, and PO 3 4 systems. Each review in this series will provide critically evaluated equilibrium data applicable to environmental waters at low ionic strength. Such values are derived from data reported in the IUPAC Stability Constants database, SC-Database [2003PET], and extrapolated to zero ionic strength (I m = 0 mol kg 1 ) using appropriate specific ion interaction theory (SIT) functions [97GRE]. A consequence of this SIT approach, which typically utilized published constants measured at I c = mol dm 3, is the generation of empirical functions that permit the calculation of log 10 K n values at intermediate values of I m as may be relevant in industrial or environmental situations. For each metal ligand combination, the review will identify the most reliable publications and stability constants; identify (and reject) unreliable stability constants; establish correlations between the selected data on the basis of ionic strength dependence, using the SIT functions; establish recommended values of β p,q,r and K s0 at 25 C ( K) and I m = 0 mol kg 1 ; identify the most reliable value of the reaction enthalpy r H m for each equilibrium reaction and establish recommended values at 25 C and I m = 0 mol kg 1 ; provide the user with the numerical relationships required to interpolate values of β p,q,r and r H m at I m >0 mol kg 1 ; and provide examples of SIT plots for β p,q,r and r H m extrapolations, and examples of distribution diagrams for binary and multicomponent systems. Literature values for metal ligand stability constants [2003PET], or formation constants [97INC], are typically determined in ionic media of nominally fixed and (comparatively) high ionic strength. The reported constants, designated by β p,q,r or K n, are valid at a single ionic strength. Most frequently, they are reported on the amount concentration (molarity) scale as equilibrium concentration products in which [species i] refers to the (amount) concentration, c, of species i in a system at equilibrium (see Appendix 1A). These concentration products are related to the standard (state) equilibrium constants, β p,q,r and K n, the equilibrium activity products, by β p,q,r = β p,q,r (lim I c 0 mol dm 3 ) and K n = K n (lim I c 0 mol dm 3 ). This is a consequence of the usual thermodynamic standard state convention for solutions: that activity coefficients of solute species approach 1 as the ionic strength (or concentration) approaches zero. As noted in the Orange Book [97INC], stability constants are as well defined thermodynamically as those referring to pure water (the equilibrium activity products); they simply refer to a different relative activity scale (standard state). In this document, to facilitate the use of the SIT functions, reported values of stability constants β p,q,r and K n were initially converted to the molality scale. The limiting values at I m = 0 mol kg 1 (β p,q,r and K n ) were then obtained by weighted linear regression against I m using the SIT equations to describe the ionic strength dependence of ion activity coefficients. The weighting (uncertainty) assigned to each value followed the guidelines in [92GRE, Appendix C]. Consistent with common practice, the quotients β p,q,r and K n are referred to as stability constants (whether defined on the molarity or molality scale), while the equilibrium activity products, β p,q,r and K n, are referred to as the standard (state) equilibrium constants ; see Appendix 1A. All reactions described in this document refer to aqueous solution, e.g.,

5 Chemical speciation of environmentally significant heavy metals 743 Hg 2+ (aq) + Cl (aq) + H 2 O HgOHCl(aq) + H + (aq) For simplicity, the suffix (aq) is not used in equations or when referring to specific species unless that species has zero net charge, in which case the phase is specified, e.g., HgOHCl(aq) and HgO(s). Further, throughout this document amount concentration is abbreviated to concentration, the units being mol dm 3 ( mol l 1, or M). 3. SUMMARY OF RECOMMENDED VALUES Tables 1 5 provide a summary of the standard equilibrium constants, reaction enthalpies, and reaction ion interaction coefficients, ε, for the formation of Hg 2+ complexes with inorganic anions. These were derived from the critical evaluation of available literature data [2003PET] and application of SIT functions. See Section 5.3 for definition of the terms Recommended (R) and Provisional (P) used in the tables and Section 5.1 for a description of the selection and evaluation process. The log β p,q,r, log K n, and log *β p,q,r values, and the reaction enthalpies, r H m, are for K, 1 bar (10 5 Pa) and infinite dilution (I m = 0 mol kg 1 ). Note that none of the values for the Hg 2+ PO 4 3 system are assigned Recommended or Provisional status, and neither of the values for the Hg 2+ SO 4 2 system is Recommended, nor applies at I m = 0 mol kg 1. See Appendix 1A for definitions of the symbols used for stability constants. Table 1 Recommended values for the Hg 2+ OH system at K, 1 bar, and I m = 0 mol kg 1. R = Recommended; P = Provisional. ε values for ClO 4 medium. Reaction Hg 2+ + H 2 O HgOH + + H + log 10 *K 1 = 3.40 ± 0.08 R ε = (0.14 ± 0.03) kg mol 1 Hg H 2 O Hg(OH) 2 (aq) + 2H + log 10 *β 2 = 5.98 ± 0.06 R ε = (0.14 ± 0.02) kg mol 1 r H m = (51.5 ± 1.8) kj mol 1 R Hg H 2 O Hg(OH) 3 + 3H + log 10 *β 3 = 21.1 ± 0.3 P HgO(s) + H 2 O Hg(OH) 2 (aq) r H m = (26.2 ± 1.8) kj mol 1 R HgO(s) + 2H + Hg 2+ + H 2 O log 10 *K s0 = 2.37 ± 0.08 R r H m = (25.3 ± 0.2) kj mol 1 R Table 2 Recommended values for the Hg 2+ Cl system at K, 1 bar, and I = 0 mol kg 1. R = Recommended; P = Provisional. ε values for NaClO 4 medium. Reaction Hg 2+ + Cl HgCl + log 10 K 1 = 7.31 ± 0.04 R ε = (0.22 ± 0.04) kg mol 1 r H m = (21.3 ± 0.7) kj mol 1 R Hg 2+ + HgCl 2 (aq) 2HgCl + log 10 K = 0.61 ± 0.03 R ε = (0.02 ± 0.02) kg mol 1 r H m = (6.5 ± 1.7) kj mol 1 R Hg Cl HgCl 2 (aq) log 10 β 2 = ± 0.07 R ε = (0.39 ± 0.03) kg mol 1 r H m = (49.1 ± 1.0) kj mol 1 R (continues on next page)

6 744 K. J. POWELL et al. Table 2 (Continued). Reaction HgCl 2 (aq) + Cl HgCl 3 log 10 K 3 = ± 0.09 R ε = (0.01 ± 0.05) kg mol 1 r H m = (0.5 ± 2.5) kj mol 1 P HgCl 3 + Cl HgCl 2 4 log 10 K 4 = 0.61 ± 0.12 R ε = (0.003 ± 0.06) kg mol 1 r H m = (10.5 ± 2.5) kj mol 1 P Hg 2+ + Cl + H 2 O HgOHCl(aq) + H + log 10 β = 4.27 ± 0.35 P Table 3 Recommended values 1 for the Hg 2+ CO 3 2 system at K, 1 bar, and I m = 0 mol kg 1. R = Recommended; P = Provisional. Reaction Hg(OH) 2 (aq) + CO 2 (g) HgCO 3 (aq) + H 2 O log 10 K = 0.70 ± 0.20 R Hg(OH) 2 (aq) + HCO 3 Hg(OH)CO 3 + H 2 O log 10 K = 0.98 ± 0.10 R Hg(OH) 2 (aq) + CO 2 (g) + H + HgHCO H 2 O log 10 K = 3.63 ± 0.10 R HgCO 3.2HgO(s)+3H 2 O 3Hg(OH) 2 (aq) +CO 2 (g) log 10 K s = ± 0.35 P 1 The value for log 10 K s refers to I c = 3.0 mol dm 3 (NaClO 4 ). Table 4 Selected values for the Hg 2+ PO 4 3 system at K, 1 bar, and I c = 3 mol dm 3 NaClO 4. Reaction Hg 2+ + HPO 2 4 HgHPO 4 (aq) log 10 K = 8.8 ± 0.2 Hg 2+ + HPO 2 4 HgPO 4 + H + log 10 K = 3.25 ± 0.2 Hg 3 (PO 4 ) 2 (s) + 2H + 3Hg HPO 2 4 log 10 *K s = 24.6 ± 0.6 (HgOH) 3 PO 4 (s) + 4H + 3Hg 2+ + HPO H 2 O log 10 *K s = 9.4 ± 0.8 HgHPO 4 (s) Hg 2+ + HPO 2 4 log 10 K s = 13.1 ± 0.1 Table 5 Selected stability constants for the Hg 2+ SO 4 2 system at K, 1 bar, and I c = 0.50 mol dm 3 NaClO 4. Reaction Hg 2+ + SO 2 4 HgSO 4 (aq) log 10 K 1 = 1.4 ± 0.1 P Hg SO 2 4 Hg(SO 4 ) 2 2 log 10 β 2 = 2.4 a a = of doubtful value, unknown uncertainty. Tables 6 and 7 report Recommended values for the protonation constants for CO 3 2 and PO 4 3. These are supplementary data that are required to complete speciation calculations on the related Hg 2+ systems. These data, and the reaction ion interaction coefficients, ε, are also derived from the critical evaluation of published stability constants [2003PET] and application of SIT functions. However, the reader is alerted to the fact that NaCl medium and different activity coefficient relationships are used for these two systems (see Sections 7.1 and 7.2).

7 Chemical speciation of environmentally significant heavy metals 745 Table 6 Recommended values for the H + CO 3 2 system at K, 1 bar, and I m = 0 mol kg 1. R = Recommended; P = Provisional. ε values for NaCl medium. Reaction H + + CO 2 3 HCO 3 log 10 K 1 = ± R ε 1 = (0.116 ± 0.002) kg mol 1 H + + HCO 3 H 2 CO 3 * 3 log 10 K 2 = ± R ε 2 = (0.092 ± 0.002) kg mol 1 1 Based on regression analysis yielding a j B = ± With a j B = 1.50, log 10 K 1 = ± 0.002, and ε = (0.078 ± 0.001) kg mol 1. See Section Based on regression analysis yielding a j B = ± With a j B = 1.50, log 10 K 2 = ± 0.001, and ε = (0.072 ± 0.001) kg mol 1. 3 [H 2 CO 3 *] = [CO 2 (aq)] + [H 2 CO 3 ]. Table 7 Recommended values for the H + PO 4 3 system at K, 1 bar, and I m = 0 mol kg 1. R = Recommended; P = Provisional. ε values for NaCl medium. Reaction H + + PO 3 4 HPO 2 4 log 10 K 1 = ± R ε 1 = (0.078 ± 0.019) kg mol 1 H + + HPO 4 2 H 2 PO 4 log 10 K 2 = ± R ε 2 = (0.061 ± 0.016) kg mol 1 H + + H 2 PO 4 H 3 PO 4 log 10 K 3 = ± R ε 3 = (0.043 ± 0.017) kg mol 1 1 Based on regression analysis yielding a j B = ± With a j B = 1.50, log 10 K 1 = ± 0.019, and ε = (0.029 ± 0.008) kg mol 1. See Section Based on regression analysis yielding a j B = ± With a j B = 1.50, log 10 K 2 = ± 0.008, and ε = (0.011 ± 0.007) kg mol 1. 3 Based on regression analysis yielding a j B = ± With a j B = 1.50, log 10 K 2 = ± 0.009, and ε = (0.033 ± 0.006) kg mol 1. The abbreviations used to describe the experimental methods are as follows. emf emf measurements using electrodes utilizing redox equilibria sol solubility determination gl ph measurement by glass electrode con conductivity dis distribution between immiscible solvents ise emf measurements using an ion selective electrode cal calorimetry sp spectrophotometry 4. Hg(II) SOLUTION CHEMISTRY Mercury has two common cations in aqueous solution, a di-ion, Hg 2 2+, composed of two singly charged ions, and a doubly charged Hg 2+. Of these, Hg(II) is the dominant form in most aqueous solutions. Diagrams of ph-potential boundaries indicate that Hg(I) is stable only within a narrow band of E H values in acid solutions [66ZOU]. The hydrolysis reactions of Hg(II) are significant at ph > 1 [86BAE], and these reactions must be taken into account in all equilibrium studies of Hg 2+ -ligand systems. At low

8 746 K. J. POWELL et al. aqueous mercury concentrations ( 0.01 mmol dm 3 ), the dominant hydrolysis species formed are monomers HgOH + and Hg(OH) 2 (aq), while Hg(OH) 3 forms at ph > 13. At higher mercury concentrations, evidence for the formation of the dimer, Hg 2 (OH) 2 2+, has been reported [62AHa, 77SJb]. 5. DATA EVALUATION METHODS 5.1 Data evaluation criteria The majority of anion complexation studies of Hg(II) have utilized the potentiometric technique and have been carried out using sodium perchlorate as the ionic medium. A few have used [Ca,Mg](ClO 4 ) 2 or [Na,K]NO 3, but the propensity of Hg 2+ to form stable chloro complexes precludes use of chloride media. In this review, stability constant and reaction enthalpy data published for 25 C and a wide range of ionic strengths (I c or I m ) have been used in weighted linear regression analyses to determine values valid at K and I m = 0 mol kg 1. Literature data have been accepted as reliable (designated reported in relevant tables), and thus included in the regression analysis, when all, or in some cases most, of the following requirements have been met: i. full experimental details are reported (solution stoichiometry, electrode calibration method, temperature, ionic strength, error analysis); ii. the equilibrium model is considered to be complete (including hydrolysis reactions); iii. iv. data are for an essentially noncomplexing medium; and the experimental method and numerical analysis are considered to have minimal systematic errors. These are not the usual IUPAC criteria for selection of published data that are used in the calculation of Recommended and Provisional values at a single ionic strength [2001PRa], but they permit utilization of a larger data set for the adopted ionic strength correlations. Most of the uncertainties reported in the literature reflect analytical and numerical precision, but do not include systematic errors. We assign an additional uncertainty to each selected value that reflects our estimation of accuracy and reliability of the experimental methods. The data selected for use in the SIT analyses for Hg 2+ complexes are recorded in Tables A2-1 through A2-15 in Appendix 2. The tables record only those data that have met our selection criteria. The column headed log 10 K (reported) contains the accepted stability constant data, on the molar scale, as published. The column headed log 10 K (accepted) contains the same data converted to the molal scale (to facilitate SIT analysis). It indicates our assigned uncertainties, which are based on the reliability and systematic uncertainties of the data. In the SIT regression analysis, the constants are weighted according to these assigned uncertainties. References that contain data rejected from our analysis are recorded in the footnotes to relevant tables. Each reference carries superscript(s) that refer to the reasons for rejection of the data according to the alphabetic list below. Reasons for rejection of specific references include: a. data are for temperature(s) other than 25 C, data cannot be corrected to 25 C, or the temperature is not defined; b. data for Hg 2+ complexation are for a medium other than (Na)ClO 4 ; c. the ionic strength has not been held constant; d. experimental details are incomplete; e. the equilibrium model is incomplete; f. electrode calibration details are missing; g. incomplete experimental data; h. the description of the numerical analysis of measurement data is incomplete;

9 Chemical speciation of environmentally significant heavy metals 747 i. the correction for competing equilibria [e.g., formation of Hg(OH) 2 (aq)] is inadequate; and j. value(s) appear to be in error when compared with results from more than one other reliable laboratory. Of this list, a and b do not necessarily question the quality of the data, but merely indicate that the data has not been used in the present review. The remaining reasons, however, place doubt in relation to the validity of the data in question. 5.2 Methods for numerical extrapolation of data to I m = 0 mol kg 1 For many reactions, equilibrium measurements cannot be made accurately, or at all, in dilute solutions (which would permit calculation of standard state values by application of simple activity coefficient relationships). Such is the case for reactions involving formation of weak complexes or ions of high charge. For these systems, precise equilibrium data can only be obtained in the presence of an inert electrolyte of sufficiently high concentration to ensure that reactant activity coefficients are reasonably constant. The associated short-range, weak, noncoulombic interactions between the reactant species and electrolyte anions or cations must be considered. They may be described in terms of ion pair formation (as required when the Debye Hückel theory or the empirical Davies equation is used for activity coefficients). Alternatively, they can be quantified by inclusion of empirical specific ion interaction coefficients, ε(i,k), within the activity coefficient expression, as in the Brønsted Guggenheim Scatchard (SIT) model [97PUI], which is adopted in this review: log 10 γ m,i = z 2 i A I m (1 + a j B I m ) 1 + Σ k ε(i,k) m k = z 2 i D + Σ k ε(i,k) m k (1) In this model, the ionic strength is expressed on the molality scale, I m. The advantage of SIT is that the activity coefficient expressions are valid over a very wide range of concentrations. In contrast, the Debye Hückel and Davies equations are limited to I m 0.03 mol kg 1 and < 0.1 mol kg 1, respectively. The ionic strength dependence of stability constants discussed in this report does not require empirical relationships more complex than eq. 1. Use of Pitzer equations would require adoption of data that have not undergone critical review. The present critical assessment of data could, however, be used to refine the Pitzer model for its application to complex, multicomponent systems. The following general reaction is assumed (omitting most charges for simplicity): pm + ql + rh 2 O M p L q (OH) r + rh + If the stability constant β p,q,r is determined in an ionic medium (containing the 1:1 electrolyte NX of ionic strength I m mol kg 1 ) and expressed in units of relative molality [m(species i)/m, where the standard molality m = 1 mol kg 1 ] it is related to the corresponding value at zero ionic strength, β p,q,r : log 10 β p,q,r = log 10 β p,q,r + plog 10 γ m (M) + qlog 10 γ m (L) + rlog 10 a(h 2 O) log 10 γ m (p,q,r) rlog 10 γ m (H + ) (2) where γ m (p,q,r) refers to the species M p L q (OH) r. Substitution of eq. 1 into 2, and the assumption that the concentration of NX is much greater than that of each reactant (such that I m = m k ), gives log 10 β p,q,r z 2 D rlog 10 a(h 2 O) = log 10 β p,q,r εi m (3) where and z 2 = (pz M + qz L r) 2 + r p(z M ) 2 q(z L ) 2

10 748 K. J. POWELL et al. ε = ε (complex, N + or X ) + rε (H +, X ) pε (M +, X ) qε (L, N + ) In this review, the term a j B is set at 1.5 kg 1/2 mol 1/2, except for the systems H + CO 2 3 and H + PO 3 4, in which it is treated as a variable in the regression analysis using eq. 3. The term log 10 a(h 2 O) is near constant for most studies of equilibrium in dilute aqueous solutions where the ionic medium is in large excess; also, log 10 a(h 2 O) 0 as I m 0 mol kg 1. For a 1:1 electrolyte (NX), this term can be calculated from the solution osmotic coefficient, Φ m, if the minor electrolyte species (the reacting ions) are neglected, whence I m m(nx): log 10 a(h 2 O) = 2m(NX)Φ m /M W (ln 10) Values for the osmotic coefficient of NaClO 4 media are available in [59ROB]; these provide the relationship log 10 a(h 2 O) = ( ± )(I m /mol kg 1 ) for NaClO 4 media at 25 C ( K), I m = 0 to 3.5 mol kg 1. The application of SIT to the selected literature stability constants involves graphical extrapolation of log 10 β p,q,r z 2 D rlog 10 a(h 2 O) to m k = 0 (or I m = 0 mol kg 1 for a system with a large excess of 1:1 electrolyte), using eq. 3. The intercept at I m = 0 mol kg 1 gives the standard equilibrium constant log 10 β p,q,r, and the slope provides the reaction ion interaction coefficient (slope = ε) as defined in eq. 3. If the regression line slope is negative, then ε is positive. Conversely, the value of the stability constant β p,q,r at a specific ionic strength, I m, can be calculated from β p,q,r if the value of the empirical parameter ε is known. Thus, this review reports values for both log 10 β p,q,r and ε. [It is noted that in eq. 3, D is a function of I m, whereas the ion interaction term (i.e., Σ k ε(i,k) m k ; eq. 1) is a function of m k ; however, m k I m for a medium containing excess 1:1 electrolyte.] The weighted linear regression analyses using SIT are represented graphically and are recorded in Appendix 3 (e.g., Fig. A3-1). The SIT regressions used values that resulted from independent analyses of experimental datum points (population values). Some values have very strong experimental bases, other have weaker grounds, some are based on a few, others on a large number of datum points, and the experimental methods also may differ. These differences were taken into account in the regression analyses by assigning appropriate weights to each value, according to [92GRE, Appendix C]. A specific regression analysis could only include values that belong to the same parent distribution, which means they must be consistent with each other. In this document, consistency was established by propagating the uncertainties of the regression results (at I m = 0 mol kg 1 ) back to high ionic strengths, viz. the dotted lines in Fig. A3-1. If the initial SIT analysis revealed values for which the uncertainty ranges did not overlap with the area between the dotted lines, they were considered to be inconsistent with the population; thus, they were removed (as outliers) or, if justified, their uncertainties were increased accordingly. For the figures in Appendix 3, the error propagation calculation that determined the confidence limits that are shown used the errors on log 10 β p,q,r, and on ε obtained in the final SIT regression. 5.3 Criteria for the assignments: Recommended and Provisional Criteria for assigning equilibrium data as Recommended or Provisional (previously Tentative ) were based on those used in more recent IUPAC critical reviews [97SJa, 97LPa, 2001PRa]. In most examples in this work, an equilibrium constant log 10 β p,q,r was classified as Recommended if the standard deviation, σ, (obtained from the SIT regression) was 0.2, and as Provisional if 0.2 σ 0.4. Values for r H m obtained from the SIT regression were assigned as Recommended if σ 2.0 kj mol 1, and as Provisional if 2.0 σ 5.0 kj mol 1. In some cases, the assignment Provisional was necessary because there were too few accepted data to allow a reliable regression analysis. However, in others more subjective judgement was used when, for example, further experimental confirmation was needed or the result is, despite good coincidence of experimental data, an unexpected one.

11 Chemical speciation of environmentally significant heavy metals EVALUATION OF EQUILIBRIUM CONSTANTS (HOMOGENEOUS REACTIONS) 6.1 The Hg 2+ OH system The speciation diagram for the Hg 2+ OH system, based on the Recommended values recorded in Table 1 for stability constants at I m = 0 mol kg 1, is shown in Fig. 1. Results outside the ph range 2 to 12 should be viewed with caution as activity coefficients may deviate significantly from 1.0. Fig. 1 Speciation diagram for the binary Hg(II) hydroxide system as obtained from the Recommended stability constants at I m = 0 mol kg 1 (Table 1). This diagram is applicable for Hg(II) concentrations 0.01 mmol dm 3. Results outside the log [H + ] range of 2 to 12 should be viewed with caution as activity coefficients deviate from Formation of HgOH + Formation of the first mononuclear hydrolysis species is described by eq. 4, Hg 2+ + H 2 O HgOH + + H + (4) Data selected for the SIT analysis, to determine the stability constant at zero ionic strength (the standard equilibrium constant) and the reaction interaction coefficient ε(4), are listed in Table A2-1, along with references and our assigned uncertainties. The selected data refer to perchlorate media and 25 C. References for rejected data are shown in the footnote, along with the reasons for rejection, designated by the superscript letters specified in Section 5.1. The uncertainties assigned to the selected data are used to determine the weight for each respective value. The weighted linear regression (Fig. A3-1) involves the expression log 10 *K 1 + 2D log 10 a(h 2 O) = log 10 *K 1 εi m that is derived from eqs. 3 and 4 ( z 2 = 2) and which yields log 10 *K 1 from the intercept and ε from the slope. It indicates reasonable consistency among the data and provides the Recommended value log 10 *K 1 (eq. 4, K) = 3.40 ± 0.08

12 750 K. J. POWELL et al. This value is identical to that selected by Baes and Mesmer [86BAE] in their review of the hydrolysis of metal ions; their value was based on a smaller and older set of stability constants. The value for ε(4) is (0.14 ± 0.03) kg mol 1. The values for ε(hg 2+,ClO 4 ) = (0.34 ± 0.03) kg mol 1 and ε(h +,ClO 4 )= (0.14 ± 0.02) kg mol 1 [97GRE] lead to ε(hgoh +,ClO 4 ) = (0.06 ± 0.05) kg mol Formation of Hg(OH) 2 (aq) Formation of the second mononuclear hydrolysis species is described by eq. 5, Hg H 2 O Hg(OH) 2 (aq) + 2H + (5) Data selected for the SIT analysis, and our estimated uncertainties, are listed in Table A2-2. References for rejected data are shown in the footnote. The selected stability constants were determined at 25 C in sodium or calcium perchlorate media of constant ionic strength. Figure A3-2 illustrates that there is a relatively large scatter of data at high ionic strength. It appears that the data obtained from calcium perchlorate media [62AHa] and from solubility experiments [61DTa] may be less reliable than the other data obtained at I c = 3.0 mol dm 3. The experimental methodology used by [62AHa] resulted in some changes in the ionic strength, whereas the work of [61DTa] potentially suffers from lack of characterization of the solid phase. The Recommended constant at zero ionic strength, derived from the weighted linear regression, is log 10 *β 2 (eq. 5, K) = 5.98 ± 0.06 The reaction ion interaction coefficient ε(5) = (0.14 ± 0.02) kg mol 1. The values for ε(hg 2+,ClO 4 ) = (0.34 ± 0.03) kg mol 1 and ε(h +,ClO 4 ) = (0.14 ± 0.02) kg mol 1 [97GRE] lead to ε(hg(oh) 2,Na +,ClO 4 ) = (0.08 ± 0.05) kg mol 1. This value is consistent with that reported by [90CIA], (0.06 ± 0.05) kg mol 1. The standard equilibrium constant recommended in this review is somewhat more positive than that evaluated by Baes and Mesmer [86BAE] (log 10 *β 2 = 6.17), but in good agreement with the value proposed by Ciavatta [90CIA] (log 10 *β 2 = 6.01) Formation of Hg(OH) 3, Hg 2 OH 3+, and Hg 2 (OH) 2 2+ Reliable stability constant data have been reported for the formation of Hg(OH) 3, Hg 2 OH 3+, and Hg 2 (OH) 2 2+ (Table A2-3). However, there are insufficient data for a SIT analysis. From the stability constant for Hg(OH) 3 determined at I m = 0 mol kg 1 [38GHa] and the stability constant for Hg(OH) 2 (log 10 *β 2 = 5.98), the stepwise stability constant log 10 *K 3 = is calculated. This species will form only in highly alkaline solutions; it is unlikely to be environmentally important. The species Hg 2 OH 3+ and Hg 2 (OH) 2 2+ will form only at relatively high Hg(II) concentrations (ca mol dm 3 ); they also are unlikely to be environmentally important. There has been some conjecture in relation to the stoichiometry of the Hg n (OH) n n+ species. Sjöberg [77SJb] found evidence for Hg 2 (OH) 2 2+, consistent with the above. In contrast, Baes and Mesmer [86BAE], in recalculating the data of Ahlberg [62AHa], concluded that the polymeric species is Hg 3 (OH) Regardless of the stoichiometry, evidence for this polymer has only been found in 3 mol dm 3 perchlorate media and, as such, it is not possible to determine a stability constant at zero ionic strength. The polymeric species Hg 4 (OH) 3 5+, also postulated [62AHa], has not been identified in any other study; as such, its existence is not accepted by this review. 6.2 The Hg 2+ Cl system The speciation diagram for the Hg 2+ Cl system, based on the Recommended values for stability constants at I m = 0 mol kg 1 (Table 2), is shown in Fig. 2, which represents the situation in which hydrolysis is suppressed (ph < 2). Results for values of log 10 [Cl ] > 2.0 should be viewed with caution as activity coefficients may deviate significantly from 1.0. Formation of the chloro complexes in aqueous solution is described by eqs. 6 9

13 Chemical speciation of environmentally significant heavy metals 751 Hg 2+ + Cl HgCl + (6) Hg Cl HgCl 2 (aq) (7) HgCl 2 (aq) + Cl HgCl 3 (8) HgCl 3 + Cl HgCl 2 4 (9) The 1:1 and 1:2 chloro complexes of Hg(II) are among the most stable of metal-chloro complexes formed. Many of the early stability constant data are not reliable because the constant ionic strength protocol was not employed. Furthermore, the simultaneous presence of HgCl 2, HgCl 3, and HgCl 2 4 was not recognized, which led to erroneous evaluations. A comprehensive investigation of the Hg(I) and Hg(II) chloride system was undertaken by Sillén in the 1940s. Since [Hg 2+ ] cannot be measured directly with a Hg electrode, due to formation of Hg(I), Sillén developed an indirect but very precise emf method based on measurement of the redox potential for Hg(II)/Hg(I) in the presence of Hg 2 Cl 2 (s). These experiments, performed at 25.0 C, I c = 0.5 mol dm 3 (NaClO 4 ) and ph = 2 (to avoid hydrolysis), are described in detail in [46SIL]. Fig. 2 Speciation diagram of the binary Hg(II) chloride system as obtained from the Recommended stability constants at I m = 0 mol kg 1, Table 2. Hydrolysis is suppressed (ph < 2 is assumed). Results for log [Cl ] > 2.0 should be viewed with caution as activity coefficients deviate from Formation of HgCl + Data selected for the SIT analysis, to determine the stability constant at zero ionic strength (the standard equilibrium constant) and the ion interaction coefficient ε for reaction 6, are listed in Table A2-4, along with our assigned uncertainties. The selected data all refer to NaClO 4 media and 25 C. The weighted linear regression (Fig. A3-3) shows that there is reasonable consistency between the data and results in the Recommended standard constant log 10 K 1 (eq. 6, K) = 7.30 ± 0.05

14 752 K. J. POWELL et al. The reaction ion interaction coefficient based on this regression is ε(6) = (0.22 ± 0.04) kg mol 1, which is in excellent agreement with that calculated from tabulated ε values [97GRE] for the reactant and product species, ε(6) = (0.18 ± 0.06) kg mol 1. Formation of HgCl + is also expressed by reaction 10 Hg 2+ + HgCl 2 (aq) 2HgCl + (10) The emf method elaborated by Sillén [46SIL, 47SIL] yields a redox titration curve that goes through a maximum of de/dc (where c is the concentration of titrant) when the concentration of HgCl + is at a maximum, because the measured emf is a function of [HgCl + ] alone. This results in precise log K values for equilibrium 10. The selected values for 25 C and (Na,H)ClO 4 media are listed in Table A2-5. With the exception of one solvent extraction (distribution) study [57MAa], all values result from application of Sillén s emf method. The weighted linear regression (Fig. A3-4) results in the recommended value of: log 10 K (eq. 10, K) = 0.61 ± 0.03 The reaction ion interaction coefficient is ε(10) = (0.02 ± 0.02) kg mol Formation of HgCl 2 (aq) Data selected for the SIT analysis of reaction 7 are listed in Table A2-6. The weighted linear regression (Fig. A3-5) shows that there is reasonable consistency between the data and results in the Recommended standard constant log 10 β 2 (eq. 7, K) = ± 0.07 The resulting reaction ion interaction coefficient is ε(7) = (0.39 ± 0.03) kg mol 1. From the reported values ε(hg 2+,ClO 4 ) = (0.34 ± 0.03) kg mol 1, ε(hgcl +,ClO 4 ) = (0.19 ± 0.02) kg mol 1, and ε(cl,na + ) = (0.03 ± 0.01) kg mol 1 [97GRE], ε(10) gives ε(hgcl 2,Na +,ClO 4 ) = (0.06 ± 0.05) kg mol 1 and ε(7) gives ε(hgcl 2,Na +,ClO 4 ) = (0.01 ± 0.04) kg mol 1. Both values are consistent with that reported by [90CIA], (0.06 ± 0.03) kg mol 1. The evaluated constants for reactions 6, 7, and 10 define an energy cycle that is consistent within experimental error. From the relationship 2log 10 K 1 (6) = log 10 K (10) + log 10 β 2 (7), we derive the Recommended standard constant log 10 K 1 (eq. 6, K) = 7.31 ± 0.04 which is consistent with that derived from the SIT analysis (Section 6.2.1) Formation of HgCl 3 and HgCl 2 4 The stepwise formation constants for the 1:3 and 1:4 Hg(II) chloro complexes are much smaller than those for the 1:1 and 1:2 complexes. Thus, comparatively large chloride concentrations are required for them to form (Fig. 2). Nevertheless, their stabilities are such that they always exist in aqueous solution simultaneously, in equilibrium with HgCl 2 (aq). Data selected for the SIT analyses for reactions 8 and 9 are listed in Table A2-7. The weighted linear regression analyses (Figs. A3-6 and A3-7) indicate excellent consistency between the data. They result in the Recommended standard constants (I m = 0 mol kg 1 ): log 10 K 3 (eq. 8, K) = ± 0.09 log 10 K 4 (eq. 9, K) = 0.61 ± 0.12 The resulting reaction ion interaction coefficients are ε(8) = (0.01 ± 0.05) kg mol 1 and ε(9) = ( ± 0.05) kg mol 1. From these, we derive new ε values: ε(na +, HgCl 3 ) = (0.05 ± 0.07) kg mol 1 and ε(na +, HgCl 2 4 ) = (0.08 ± 0.09) kg mol 1.

15 Chemical speciation of environmentally significant heavy metals 753 The first value is in the range expected for +1/ 1 interactions, whereas for the +1/ 2 category, ε values are usually negative, cf. [92GRE, Table B.4]. The SIT analysis used only data obtained for NaClO 4 media and 25 C. However, the results of the refractometric investigation of Barcza [76BAb] in 1.0 mol dm 3 NaNO 3 solution (log 10 K 3 = 0.89), and Dubinskii and Shul man s log 10 K 3 value in 0.4 mol dm 3 HClO 4 solution [70DSe] (0.96), agree well with our selected values Other binary Hg(II) chloro complexes The formation of Hg(II) chloro complexes higher than 1:4 has not been observed in aqueous solution. However, Linhart [15LIa, 16LIa] suggested the existence of the dimeric complexes Hg 2 Cl 4 (aq), Hg 2 Cl 5, and Hg 2 Cl 6 2 from distribution measurements with total Hg(II) concentrations up to 0.29 mol dm 3. Tourneux [34TOa] also reported the formation of dimeric anionic complexes, but the ionic strength was varied substantially and it is difficult to distinguish medium effects from weak complexation. Ciavatta and Grimaldi [68CGb] varied the total Hg(II) concentration from to 0.19 mol dm 3 [I c = 1.0 mol dm 3 (Na)ClO 4 ] and found no evidence for polynuclear Hg(II) Cl complexes. It is thus inferred that the binary Hg(II) chloride system in acidic aqueous solution is fully characterized by the complexes HgCl +, HgCl 2 (aq), HgCl 3, and HgCl The Hg 2+ OH Cl system: Formation of HgOHCl(aq) Sjöberg [77SJb] made a detailed investigation of the ternary system Hg 2+ OH Cl, varying the component concentrations over large ranges. He found evidence for the formation of HgOHCl(aq) in solution at ph 3 to 9 and for log 10 [Cl ] in the range 1 to 7 (reaction 11): HgCl 2 (aq) + H 2 O HgOHCl(aq) + Cl + H + (11) At a total concentration [Hg] T > 10 mmol dm 3, the polynuclear complexes Hg 2 (OH)Cl + 2 and Hg 3 (OH) 2 Cl 3+ also formed. Other authors investigating the hydrolysis of HgCl 2 [65PIa, 68CGa, 76CGb] also observed the formation of HgOHCl(aq). Table A2-8 records the reported stability constants. Considering the different ionic strengths, and the possible systematic uncertainties that are undoubtedly larger than those reported, the three values for HgOHCl(aq) formation are remarkably consistent. Extrapolation of the stability constants to zero ionic strength is not possible because of the scarcity of data. However, the importance of reaction 11 demands that an estimate be made for log 10 β for reaction 12 at I m = 0 mol kg 1. Hg 2+ + Cl + H 2 O HgOHCl(aq) + H + (12) This can be effected by use of eq. 3, an estimated value for ε(12) in NaClO 4 media: ε(12) = ε(hgohcl,na +,ClO 4 ) + ε(h +,ClO 4 ) ε(hg 2+,ClO 4 ) ε(cl,na + ) (13) and the value calculated for log β(12) at I m =3.503 m [log β(12) = 4.12], derived from the data in Tables A2-6 and A2-8. In eq. 13, the value for ε(hgohcl,na +,ClO 4 ) is not known, but can be estimated from ε (HgOHCl,Na +,ClO 4 )= 1 /2 [ε (Hg(OH) 2,Na +,ClO 4 )+ ε(hgcl 2,Na +,ClO 4 )]. Based on the average of ε values derived in this work, and those from [90CIA], this yields ε (HgOHCl,Na +,ClO 4 ) = (0.01 ± 0.09) kg mol 1, and hence ε(12) = (0.24 ± 0.10) kg mol 1. From eq. 3, the value calculated for log 10 β (eq. 12, K) is 4.27 ± 0.35.

16 754 K. J. POWELL et al. 6.4 The Hg 2+ CO 3 2 system Figures 3a and 3b present speciation diagrams for the Hg 2+ H + CO 2 system, based on a CO 2 fugacity f(co 2 ) of 370 µbar and 1 bar, respectively, and our Recommended standard constants at I m = 0 mol kg 1 (Tables 1 and 3). Fig. 3 Speciation diagrams for the Hg 2+ H + CO 2 system as obtained from the selected stability constants reported in Tables 1 and 3 and calculated for (a) f(co 2 ) = 370 microbar and (b) f(co 2 ) = 1 bar. These diagrams are applicable for Hg(II) concentrations below 0.01 mmol dm 3. Results outside the log [H + ] range of 2 to 12 should be viewed with caution as activity coefficients deviate from 1.0.

17 Chemical speciation of environmentally significant heavy metals 755 Equilibrium constants for the formation of HgOH + and HgCO 3 (aq) are of a similar magnitude. However, the ratio of concentrations [Hg(OH) 2 (aq)]/[hgoh + ] is >1 over a wide ph range; thus, experimental evaluation of Hg 2+ CO 2 3 equilibria requires accurate corrections for the coformation of Hg(OH) 2 (aq), the predominant Hg(II) hydroxy complex in the ph region relevant to environmental systems. The only reported equilibrium studies of aqueous Hg(II) interactions with carbonate are those of Hietanen and Högfeldt [76HHa, 76HHb] at 25 C (3.0 mol dm 3 NaClO 4 ) and of Bilinski et al. [80BMb] at 25 C (0.5 mol dm 3 NaClO 4 ). The former results, summarized in Table A2-9, are somewhat lacking in precision, but are regarded as more reliable than the latter. Extrapolation to zero ionic strength is not possible. However, the derived isocoulombic reactions shown in Table A2-10 should have minimal dependence on ionic strength. Consequently, the isocoulombic equilibrium constants shown in Table A2-10, based on the Table A2-9 values, have been used as Recommended values appropriate to I m = 0 mol kg 1, 25 C, and 1 bar total pressure. To assess the influence of CO 2 on Hg(II) speciation in natural waters, equilibrium with the atmosphere [CO 2 fugacity, f(co 2 ), equal to 370 µbar] is assumed. From the data in Table A2-10 for reactions 14 and 15, I c = 3.0 mol dm 3 : HgOH + + CO 2 (g) HgHCO + 3 (14) Hg(OH) 2 (aq) + CO 2 (g) HgCO 3 (aq) + H 2 O (15) the following calculated concentration ratios, relevant to natural waters, are independent of ph: [HgHCO + 3 ]/[HgOH + ] = (reaction 14) and [HgCO 3 (aq)]/[hg(oh) 2 (aq)] = (reaction 15). For reaction 16, Hg(OH) 2 (aq) + HCO 3 Hg(OH)CO 3 + H 2 O (16) [Hg(OH)CO 3 ]/[Hg(OH) 2 (aq)] = [HCO 3 ]; this indicates that Hg(OH)CO 3 is significant relative to Hg(OH) 2 (aq) only at very high HCO 3 concentrations. The data in Table A2-10 provide relationships between the concentrations of HgHCO + 3, HgCO 3 (aq), and Hg(OH)CO 3. Since [HgHCO 3 ]/[HgCO 3 ] = [H + ], it follows that HgCO 3 (aq) is a dominant species above ph 4.3, independent of f(co 2 ). Since [Hg(OH)CO 3 ]/[HgCO 3 ] = [HCO 3 ]/f(co 2 ), the species Hg(OH)CO 3 can be dominant relative to HgCO 3 (aq) in natural waters. For f(co 2 ) = 370 µbar, the ratio [Hg(OH)CO 3 ]/[HgCO 3 ] is greater than unity when the HCO 3 concentration exceeds approximately 8 µmol dm The Hg 2+ PO 4 3 system For the Hg 2+ H + PO 3 4 system, there are a limited number of data at I c = 3.0 mol dm 3 NaClO 4. A SIT analysis is not possible; the selected data are neither Recommended nor Provisional (Table 4). Potentiometric study of Hg 2+ PO 3 4 equilibria is confounded by the low solubility of HgHPO 4 (s), Hg 3 (PO 4 ) 2 (s), and (HgOH) 3 PO 4 (s). A single paper [75QDa] reports the formation of two rather stable, water-soluble Hg(II)-phosphate complexes, HgHPO 4 (aq) and HgPO 4 at 25 C, 3.0 mol dm 3 NaClO 4, reactions 17 and 18 (Table 4). Hg 2+ + HPO 2 4 HgHPO 4 (aq) (17) Hg 2+ + HPO 2 4 HgPO 4 + H + (18) By using the protonation constants for phosphate ion under identical conditions [69BSb], stability constants can be derived for reactions 19 and 20: Hg 2+ + H + + PO 3 4 HgHPO 4 (aq) (19)

18 756 K. J. POWELL et al. Hg 2+ + PO 3 4 HgPO 4 (20) viz. log 10 β (eq. 19, K) = ± 0.2 and log 10 Κ 1 (eq. 20, K) = 14.1 ± 0.2, respectively. Despite the lack of Recommended values, the importance of phosphate as a water pollutant necessitates further discussion. At sufficiently high concentrations (e.g., total concentrations [Hg 2+ ] T = mol dm 3 and [PO 3 4 ] T = mol dm 3 ) Hg(II) speciation is dominated by the two watersoluble complexes, HgHPO 4 and HgPO 4, over a wide ph range (Fig. 4), with Hg(OH) 2 (aq) dominating at ph > 7. The three Hg(II)-phosphate solid phases form at higher concentrations of Hg(II) or phosphate, with HgO(s) forming at ph > 9.4 (Fig. 5). Fig. 4 Speciation diagram for the Hg 2+ H + PO 4 3 system (total concentrations [Hg 2+ ] T = mol dm 3 and [PO 4 3 ] T = mol dm 3 ) as obtained from the selected stability constants reported in Table 4 (I c = 3 mol dm 3 NaClO 4 ). Fig. 5 Speciation diagram for soluble and insoluble species in the Hg 2+ H + PO 4 3 system (total concentrations [Hg 2+ ] T = mol dm 3 and [PO 4 3 ] T = mol dm 3 ) as obtained from the selected stability and solubility constants reported in Table 4 (I c = 3 mol dm 3 NaClO 4 ). Solid phases shown by bold lines.

19 Chemical speciation of environmentally significant heavy metals The Hg 2+ SO 4 2 system For the interactions between cations and strongly hydrated polyvalent anions such as SO 2 4, CO 2 3, and PO 3 4, special care must be taken in the interpretation of experimental data. This is because such systems may involve the formation of both inner-sphere ( contact ) and outer-sphere ( solvent separated and solvent shared ) complexes. A detailed consideration shows that under such circumstances, the common spectroscopic methods (UV vis, NMR, and Raman) probe different equilibrium processes from those determined by traditional methods such as potentiometry and conductivity. Thus, constants obtained for such systems by different experimental methods may not be comparable. This is discussed in more detail in Appendix 1B, using SO 2 4 as an example. Only three papers report quantitative measurements for the Hg 2+ H + SO 2 4 system, all at similar I c, so a SIT analysis is not possible. The selected data (Table 5), neither of which is Recommended, refer to I c = 0.5 mol dm 3 (NaClO 4 ). Stability constants listed in more recent publications [see, e.g., 93MOR] appear to be derived from these papers. The small stability constants indicate that formation of sulfate complexes will not be a major feature of Hg(II) speciation in typical environmental fresh waters (total concentration [SO 2 4 ] T 10 4 M), even though the constants should increase considerably with decreasing I [2001KRA]. None of the published data for Hg 2+ SO 2 4 complexation is fully satisfactory. Surprisingly, no quantitative IR or Raman study of this system appears to have been made [2002RUD]. The investigation considered most reliable is that of Infeldt and Sillén [46ISa] who used mercury electrode [Hg/Hg(I),Hg(II)] potentiometry to measure K 1 and β 2. The values obtained (Table A2-11) were based on the stability constants for the Hg(I) SO 2 4 system, determined in the same paper. Because of solubility problems, only a limited range of concentrations could be investigated. Using the UV band of Hg 2+ (aq) in a study of the substitution kinetics of Co(III) complexes, Posey and Taube [57PTa] reported just three spectrophotometric measurements of K 1 (reaction 23): Hg 2+ + SO 2 4 Hg(SO 4 )(aq) (21) Although conditions were favorable, with the ratio of total concentrations [SO 2 4 ] T /[Hg 2+ ] T 30, no higher order complexes were detected. The values reported for K 1 [46ISa, 57PTa] are in fair agreement, but their average (Table 5) must be regarded as Provisional, pending further investigation. The reported value of β 2 [46ISa], although qualitatively confirmed [57KSb], is considered doubtful because of: (i) possible activity coefficient variation resulting from significant replacement of the NaClO 4 medium by Na 2 SO 4 at constant I, (ii) the failure of [57PTa] to detect such a complex under conditions favorable to its formation, and (iii) the absence of evidence for such species in related, but much better characterized, metal(ii) SO 2 4 systems. 7. SPECIATION IN 2-COMPONENT SYSTEMS: H + L Examples of speciation calculations for the selected Hg 2+ -ligand systems are given in Figs For those ligands that are the conjugate bases of weak acids (CO 3 2, PO 4 3 ), the calculations require values for the ligand protonation constants under the same conditions of ionic strength and medium. To this end, this review also presents an analysis of available data for the required ligand protonation reactions. 7.1 The H + CO 3 2 system Table 6 records the Recommended standard stability (protonation) constants for the CO 3 2 protonation reactions at 25 C and I m = 0 mol kg 1, based on weighted linear regression analyses of an extensive series of selected data for NaCl media (Table A4-1a). The data for NaClO 4 media are fewer (Table A4-1b) and provide a much inferior regression.